1. Field of the Invention
The present invention relates to a non-oriented magnetic steel sheet primarily used for iron cores for electric apparatuses and to a manufacturing method therefor.
2. Description of the Related Art
Recently, in the worldwide trend toward energy saving, typically electric power energy saving, compact electric apparatuses having increased efficiency have been desired. In this connection, in view of the miniaturization of electric apparatuses, compact iron cores have also been desired. In order to respond to these desires, non-oriented magnetic steel sheets, primarily used as materials for iron cores for use in electric apparatuses, are required to have a low iron loss and increased efficiency.
Conventionally, in order to reduce iron loss of non-oriented magnetic steel sheets, methods for increasing the content of silicon (Si), aluminum (Al), manganese (Mn), and the like are generally employed. These methods have as an object to decrease eddy-current loss by increasing electric resistance in a steel sheet. However, in these methods, non-magnetic components are increased, and as a result, there is a problem in that a decrease in magnetic flux density cannot be avoided.
A method is also known in which, in addition to an increase of the content of Si or Al, the content of carbon (C) and/or sulfur (S) is decreased, and an alloy component such as boron (B) or nickel (Ni) is increased. The addition of B is disclosed in Japanese Unexamined Patent Application Publication No. 58-15,143. The addition of Ni is disclosed in Japanese Unexamined Patent Application Publication No. 3-281,758. In the methods in which an alloy component is added, iron loss is improved, but improvement in magnetic flux density is not significant. In addition, in the methods described above, workability of a steel sheet is degraded since hardness thereof is increased concomitant with an increase in the content of alloy component. As a result, a non-oriented magnetic steel sheet cannot be fabricated for use in electric apparatuses in some cases. Accordingly, the applications thereof are very limited, and hence, broad application of the steel sheets is difficult.
There is another method for improving magnetic properties in which the degree of crystallographic directional concentration (texture of the steel sheet) is improved by changing a manufacturing process. For example, in Japanese Unexamined Patent Application Publication No. 58-181,822, a method is disclosed in which a steel containing 2.8 to 4.0 weight % Si and 0.3 to 2.0 weight % Al is hot-rolled in the range from 200 to 500xc2x0 C. so as to grow in the {100} less than UVW greater than  direction. In Japanese Unexamined Patent Application Publication No. 3-294,422, a method is disclosed in which a steel containing 1.5 to 4.0 weight % Si and 0.1 to 2.0 weight % Al is hot-rolled, is annealed between 1,000 to 1,200xc2x0 C., and is then cold-rolled with the reduction in thickness of 80 to 90% so as to grow in the {100} less than UVW greater than  direction. However, the improvement in magnetic properties by these methods described above is not significant. For example, in example 2 of Japanese Unexamined Patent Application Publication No. 58-181,822, the magnetic flux density and iron loss of a 0.35 mm-thick finished steel sheet, which contains 3.4 weight % Si, and 0.60 weight % Al, are 1.70 T of B50 and 2.1 W/kg of W15/50, respectively. In Japanese Unexamined Patent Application Publication No. 3-294,422, the magnetic flux density and iron loss of a 0.50 mm-thick finished steel sheet, which contains 3.0 weight % Si, 0.30 weight % Al, and 0.20 weight % Mn, are 1.71 T of B50 and 2.5 W/kg of W15/50, respectively.
In addition, there have been proposals to change the manufacturing process. However, in every proposed technique, satisfactory finished steel sheets having a low iron loss and a high magnetic flux density have not been obtained.
Objects of the present invention are to provide a non-oriented magnetic steel sheet having magnetic properties, such as a low iron loss and a high magnetic flux density, which are far superior to those obtained by conventional techniques, and to provide a manufacturing method therefor.
In order to realize a non-oriented magnetic steel sheet having a low iron loss and a high magnetic flux density at the same time, the inventors of the present invention performed intensive research on the problems in the conventional techniques. As a result, a novel non-oriented magnetic steel sheet and a manufacturing method therefor were developed.
A non-oriented magnetic steel sheet having a low iron loss and a high magnetic flux density, according to the present invention, comprises from about 1.5 to about 8.0 weight % Si, from about 0.005 to about 2.50 weight % Mn, and not more than about 50 ppm each of carbon (C), sulfur (S), nitrogen (N), oxygen (O), and boron (B), wherein a parameter  less than xcex93 greater than  of a crystal orientation represented by an equation (1) is about 0.200 or less,                                           ⟨            Γ            ⟩                    =                                    ∑                              j                =                1                            n                        ⁢                                          V                j                            ⁢                                                ∑                                      i                    =                    1                                    m                                ⁢                                                      (                                                                                            u                          ij                          2                                                ⁢                                                  v                          ij                          2                                                                    +                                                                        v                          ij                          2                                                ⁢                                                  w                          ij                          2                                                                    +                                                                        w                          ij                          2                                                ⁢                                                  u                          ij                          2                                                                                      )                                    /                  m                                                                    ,                            (        1        )            
in which (uij, vij, wij) is an ith vector (i=1,2, . . . m; j=1,2, . . . n; uij2+vij2+wij2=1) which is obtained from a crystal grain j having a crystal orientation represented by (hkl) less than uvw greater than , and which is parallel to a direction inclined 90xc3x97i/(mxe2x88x921) degrees on a rolled surface from a rolled direction to a direction perpendicular thereto, and Vj is an areal ratio of the crystal grain J to the total area of measured crystal grains.
In the non-oriented magnetic steel sheet having a low iron loss and a high magnetic flux density, according to the present invention, an average crystal grain diameter is preferably from about 50 to about 500 xcexcm, and an areal ratio of crystal grains on a surface of the steel sheet is preferably about 20% or less, in which crystal plane orientations of the crystal grains are within 15xc2x0 from the  less than 111 greater than  axis. In addition, in the non-oriented magnetic steel according to the present invention, from about 0.0010 to about 0.10 weight % Al is preferably present, the  less than xcex93 greater than  is preferably about 0.195 or less, and from about 0.01 to about 0.50 weight % antimony (Sb) is also preferably present. Furthermore, at least one member selected from the group consisting of from about 0.01 to about 3.50 weight % nickel (Ni), from about 0.01 to about 1.50 weight % tin (Sn), from about 0.01 to about 1.50 weight % copper (Cu), from about 0.005 to about 0.50 weight % phosphorus (P), and from about 0.01 to about 1.50 weight % chromium (Cr) is preferably contained in the non-oriented magnetic steel according to the present invention.
A method for manufacturing a non-oriented magnetic steel sheet having a low iron loss and a high magnetic flux density, according to the present invention, comprises steps of preparing a molten steel containing from about 1.5 to about 8.0 weight % Si, from about 0.005 to about 2.50 weight % Mn, and not more than about 50 ppm each of S, N, O, and B, a forming step of forming a slab from the molten steel, hot rolling the slab, annealing the hot-rolled steel sheet, cold rolling comprising a step of cold rolling the annealed steel sheet one time or a step of cold rolling the annealed steel sheet at least two times with an interim annealing step therebetween so as to have a final thickness, annealing the cold-rolled steel sheet for recrystallization, and optionally performing coating for insulation, wherein the carbon content is controlled to be about 50 ppm or less at the preparation of a molten steel or prior to annealing cold-rolled sheet and in annealing the hot-rolled sheet , said annealing is performed in a temperature range from about 800 to about 120xc2x0 C. and the temperature is subsequently decreased from about 800 to about 400xc2x0 C. at a rate of from about 5 to about 80xc2x0 C./second. In the method for manufacturing a non-oriented magnetic steel sheet according to present invention, in annealing cold-rolled sheet, the rate of increase in temperature is preferably set to be about 100xc2x0 C./hour or less in a range of from about 700xc2x0 C. and above so that the temperature reaches a range from about 750 to about 1,200xc2x0 C. It is also preferable that, in annealing cold-rolled sheet, the rate of increase in temperature be set to be about 2xc2x0 C./second or more in a range from about 500 to about 700xc2x0 C., the temperature be increased to about 700xc2x0 C. or above so as to complete recrystallization of the steel sheet, the temperature be then decreased to a range of about 700xc2x0 C. and below and again increased, and subsequently the rate of increase in temperature be set to be about 100xc2x0 C./hour or less in the range of from about 700xc2x0 C. and above so that the temperature reaches a range from about 750 to about 1,200xc2x0 C. In the method for manufacturing a non-oriented magnetic steel sheet according to the present invention, an average crystal grain diameter prior to final cold rolling is preferably set to be about 100 xcexcm or more, and the final cold rolling is preferably performed at from about 150 to about 350xc2x0 C. in at least one pass thereof. In addition, the molten steel may further comprise from about 0.0010 to about 0.10 weight % Al, and from about 0.01 to about 0.50 weight % Sb. Furthermore, the molten steel preferably further comprises at least one member selected from the group consisting of from about 0.01 to about 3.50 weight % Ni, from about 0.01 to about 1.50 weight % Sn, from about 0.01 to about 1.50 weight % Cu, from about 0.005 to about 0.50 weight % P, and from about 0.01 to about 1.50 weight % Cr.